TM RFD 007                                20 SEPTEMBER 1982


                TECHNICAL MEMO


        PRODUCT: RFD DISK DRIVES
        SUBJECT: OPTION TABLE


        Percom RFD disk drives for the Atari home computers use an
        intelligent disk controller.  In order to properly handle its
        functions, a ROM chip containing an Option Table has been developed.
        This Option Table is located in a 1K RAM chip on the controller
        board, and can be accessed by request from the computer.

        Below are tables which will help the programmer in understanding the
        calls on the Percom controller.  Of particular note are the bytes
        which handle the density of the controller.

        The Read Option and Write Option commands pertain to the 12 byte
        Option Table.  There is one table set up for each drive.

        To access these tables, see the SAMPLE program on the Percom
        Software Support diskette that comes with Percom Double Density
        drive systems.


        ==============================================================
                       INDIVIDUAL COMMANDS TABLE               TABLE 1

        FORMAT         21(hex)   FORMAT DISKETTE
        RDOPT          4E        READ OPTION TABLE FOR A DRIVE (TABLE 2)   
        WROPT          4F        WRITE OPTION TABLE FOR A DRIVE (TABLE 2) 
        WRSECT         50        WRITE SECTOR WITHOUT VERIFY 
                       51        UNDEFINED   
        RESECT         52        READ SECTOR 
        RESTCM         53        READ STATUS MESSAGE (TABLE 3)
                       54        UNDEFINED 
                       55        UNDEFINED 
                       56        UNDEFINED 
                       57        WRITE SECTOR WITH VERIFY

        ===============================================================

        ===============================================================
                   OPTION TABLE                                  TABLE 2

        All 2-byte values are in MSB,LSB format, unlike the 6502 practice
        which uses LSB,MSB format.

          BYTE(S)        OPTION
           0        Number of tracks (This is a literal value)
           1        Step Rate (Standard 17xx FDC coding)               












                       value     rate 
                       -----     ---- 
                        00        30 msecs.
                        01        20 msecs.
                        02        12 msecs.
                        03        06 msecs.

         2,3        Sectors per track (This is a literal value)

           4        Number of Sides     0= 1 side
                                        1= 2 sides
                    any other values will cause unpredictable results

           5        Density             00= Single Density
                                        04= Double Density

                    any other values will cause unpredictable results

         6,7        Sector Size (bytes per sector.  This is a literal
                    value)

           8        Drive Present       00= Drive is not present
                    any other value indicates a drive is present

           9        ACIA      not currently implemented

        Bytes 10 and 11 are reserved for future expansion

        ==============================================================
                             DRIVE STATUS                       TABLE 3

           BYTE            USE

           0    SYSTEM STATUS OF DRIVE
         
                       value     status
                       -----     ------
                       32(dec)   sector size     0=128  1=256
                       16        motor on?
                       8         write protect
                       4         FDC error
                       2         data frame error 
                       1         cmd frame error 

           1    FDC status

           2    Timeout for format- How long to wait for response to
                format

           3    Current track where read/write head is located

        ==============================================================

                             -END-